A coil's lifespan is really determined by the sweetness or other factors that cause it to gunk up or become sticky on the coil. A lower resistance heats up faster, but typically with a regulated PV, it doesn't create much more heat than a higher resistance. Wattage is the measurement of power/current and voltage and is also a reference for friction energy created in the circuit. If you're using the same Power/Wattage outputs, you will create the same heat from friction in different coils.
A lot of liquids turn darker as more air is introduced through out them, but you may notice that liquids get darker in tanks. If you've ever used cartomizers, you'll notice that when you blow them out to change your tank that the liquid is extremely dark. This is from the greater amount of airflow near the coil, and the heat from the coil is also a large catalyst in darkening liquid in a tank. If the coil wasn't near the flow of liquid, it would not let small amounts of this darkening liquid back into the tank itself. Wicks aren't one direction valves, unfortunately...
With clearomizers, it's always a good idea to clean your heads before reloading your tank. I know some people on here that make a nightly habit of washing their coils, and they love their clearomizers... I use cartomizers, but if the wick wasn't so compact and easy to properly rinse, I might make a habit of cleaning them as well. Getting a week+ out of a cartomizer is a miracle for me.
I rarely fill my tanks up all the way.
